DE102020206798B4 - Method for the automated measurement of a characteristic map of an internal combustion engine on a test bench and control unit for controlling a test bench - Google Patents

Abstract

Method for the automated measurement of a characteristic map of an internal combustion engine (3) on a test stand (1), wherein a) for the characteristic map to be measured, which is spanned by a combustion center and a combustion air/fuel ratio, i.e. a lambda value (λ), a start value pair ( 7) for the center of combustion and the lambda value (λ) in an estimated map center of the map at a predetermined concentration of pollutants in the exhaust gas of the internal combustion engine (3) is determined, with b) starting from the start value pair (7) the center of combustion gradually retarded and the lambda value (λ) are changed to lower values under the condition that the predetermined pollutant concentration is maintained, with corresponding values for the combustion center and the lambda value (λ) being stored untilc) a first limit (11) is reached, with d) starting from the Start value pair (7) progressively the center of combustion early and the Lambda value (λ) is changed to higher values under the condition that the predetermined pollutant concentration is maintained, with corresponding values for the combustion center and the lambda value (λ) being stored untile) a second limit (13) is reached, the combustion center at the second limit (13) is stored as the limit focal point of combustion (15), whereby f) the lambda value (λ) is increased in steps until a third limit (17) in a first pair of limit values (19) from the focal point of combustion and the lambda value ( λ) is achieved, with g) starting from the first pair of limit values (19) at the third limit (17), the center of combustion gradually being retarded and the lambda value (λ) being changed to lower values along the third limit (17), with corresponding Values for the combustion center and the lambda value (λ) are stored until the first limit (11) is reached, where h) along the first limit (11) Schr the lambda value (λ) is changed to lower values and if there is a deviation from the first limit (11), the center of combustion is varied in order to remain at the first limit (11), with corresponding values for the center of combustion and the lambda value (λ) being stored, until the second limit (13) is reached in a second pair of limit values (21) from the center of combustion and the lambda value (λ), whereby i) starting from the second pair of limit values (21), the center of combustion progressively advances and the lambda value (λ ) are changed to higher values along the second boundary (13), corresponding values of the center of combustion and the lambda value (λ) being stored until the third boundary (17) is reached, wherein j) stepping along the third boundary (17) the center of combustion retarded and the lambda value (λ) are changed to lower values, with corresponding values of the combustion center and the lambda value (λ) being stored be made until the center of combustion on the third boundary (17) is equal to the limit center of combustion (15).

Description

The invention relates to a method for the automated measurement of a characteristic map of an internal combustion engine on a test bench and a control device for controlling a test bench of an internal combustion engine.

Test benches for measuring characteristic diagrams of internal combustion engines are typically operated manually by a so-called test bench driver. The test bench driver attempts to scan the limits of the map on the one hand on the basis of previous knowledge about the expected behavior of the internal combustion engine and on the other hand on the basis of the intuition acquired through experience or instruction. However, the quality of the data collected is directly dependent on the cognitive abilities and the condition of the test bench driver on the day.

Out of AT 518 676 A1 a method for calibrating a technical system, in particular an internal combustion engine, emerges, with a specific procedure being proposed in order to model a data envelope around a characteristic map. Methods for determining engine characteristic diagrams or for calibrating control units of a combustion engine also come from Kuder, J. [et al.]: Parameter optimization on Otto engines with direct injection, In: MTZ, Edition 61, 2000, pp. 380-381 - ISSN 0024 -8525; Unland, S. [et al.]: New, efficient application methods for the physically based engine control ME7, In: MTZ, Edition 59, 1998, pp. 748-749 - ISSN 0024-8525; and Knödler, K. [et al.]: Model-based online optimization of modern combustion engines - Part 2: Limits of the drivable search space, In: MTZ, Edition 64, 2003, pp. 520-526 - ISSN 0024-8525.

The invention is based on the object of creating a method for automatically measuring a characteristic map of an internal combustion engine and a control device for controlling a test stand for an internal combustion engine, the disadvantages mentioned occurring at most to a reduced extent, and preferably not occurring.

The object is achieved by providing the present technical teaching, in particular the teaching of the independent claims and the embodiments disclosed in the dependent claims and the description.

The object is achieved in particular by creating a method for the automated measurement of a characteristic map of an internal combustion engine on a test bench, in which a) a start is made for the characteristic map to be measured, which is spanned by a combustion center and a combustion air-fuel ratio, hereinafter referred to as the lambda value value pair, comprising a start value for the combustion center and a start value for the lambda value, is determined in an estimated map center of the map such that when the internal combustion engine is operated with the start value pair, there is a predetermined pollutant concentration in the exhaust gas of the internal combustion engine. The internal combustion engine is operated with the starting value pair. Starting from the starting values--during operation of the internal combustion engine--the center of combustion is gradually changed to late and the lambda value to lower values in such a way that the predetermined pollutant concentration is maintained. Values for the center of combustion and the lambda value for which this condition is met are stored.

This stepwise change in the combustion center and the lambda value according to b) is repeated until c) a first limit is reached in the map to be measured.

Then d) starting from the pair of starting values, the center of combustion is gradually advanced and the lambda value is changed to higher values in such a way that the predetermined pollutant concentration is retained. Again, those values for the center of combustion and the lambda value are stored for which this condition is met.

The stepwise change of the combustion center and the lambda value according to d) is repeated until e) a second limit is reached in the map to be measured, the combustion center at the second limit being stored as the limit combustion center.

The map is first scanned in a central area along the predetermined pollutant concentration, starting from the start value pair, first in step b) in a first direction and then in step d) in a second, opposite direction until a predetermined limit is reached in each case is.

Starting from the pair of values at which the second limit was reached and which includes the limit center of combustion, f) the lambda value is now increased step by step until a third limit in a first limit value pair, comprising a value for the center of combustion and a value for the lambda value, is reached. In this case, in particular, the center of combustion is kept constant at the value of the limit center of combustion.

The first pair of limit values thus includes in particular the limit center of combustion as a value for the center of combustion.

Starting from the first pair of limit values at the third limit, g) the center of combustion is gradually retarded and the lambda value is changed to lower values along the third limit, corresponding values for the center of combustion and the lambda value being stored. This means in particular that when the combustion center and the lambda value are varied, care is taken to ensure that the map is scanned along the third limit, with only those values for the combustion center and the lambda value being stored that actually lie on the third limit. The change in the center of combustion and the lambda value according to g) is continued until the first limit is reached again—at the level of the third limit. In particular, a pair of limit values is achieved in which the third limit and the first limit intersect.

The lambda value is now h) gradually changed to lower values along the first limit. If the operating point of the internal combustion engine deviates from the first limit, the center of combustion is varied in order to remain at the first limit. The map is thus scanned along the first boundary. Corresponding values of the center of combustion and the lambda value that lie on the first limit are stored. The corresponding change in the lambda value and, if necessary, the combustion focus according to h) is repeated until the second limit in a second limit value pair, comprising a value for the combustion focus and a value for the lambda value, is reached. In the second pair of limit values, the first limit and the second limit in particular now intersect.

Starting from the second pair of limit values, the center of combustion is now i) gradually changed further and the lambda value is changed to higher values along the second limit. The change thus takes place in such a way that the characteristics map is scanned along the second boundary. In this case, such values of the center of combustion and of the lambda value that lie on the second limit are stored. The corresponding change in the center of combustion and the lambda value according to i) is repeated until the third limit is reached.

The third limit is reached in particular in a pair of limit values in which the second limit and the third limit intersect.

Then—particularly starting from this pair of limit values—j) the center of combustion is gradually changed along the third limit to late and the lambda value to lower values. The map is thus again sampled along the third limit, albeit beyond the center of combustion limit. Those values of the center of combustion and of the lambda value that lie on the third limit are stored. The corresponding change in the combustion center and the lambda value according to j) is repeated until the combustion center again—on the third limit—is equal to the limit combustion center.

As a result, the map is scanned once, starting from the start value pair, along a map line that is defined by the predetermined pollutant concentration, in a central area and once circled along its boundaries. The map is delimited by the first limit, the second limit and the third limit, the second limit having a first point of intersection with the first limit and a second point of intersection with the third limit, and with the rest of the first limit and the third Intersect border in another intersection. The map thus has an enclosed inner area which is delimited on all sides by the first limit, the second limit and the third limit. In particular, the second boundary has a course that is more strongly curved or kinked in some areas, while the first boundary and the third boundary essentially run along straight lines or lines that are less curved over the entire course.

The method proposed here advantageously enables automated measurement of the characteristics map on the basis of a standardized, rational procedure. The method can advantageously be carried out on the control unit of the test bench without the test bench operator having to intervene. Thus, the result with regard to the measurement of the map is no longer dependent on the subjective characteristics of the test bench driver. In particular, the result is reproducible.

The focal point of combustion is understood to mean, in particular, a point in time or a crankshaft angle at which 50% of the fuel mass used or 50% of the chemical energy introduced into a combustion chamber of the internal combustion engine in the form of fuel has been converted. The fact that the center of combustion is changed early means in particular that it is relative to a top dead center position of a piston internal combustion engine, preferably designed as a reciprocating piston engine, which separates a compression stroke from a working stroke, and which is also referred to as ignition TDC, is changed in the direction of an earlier point in time or an earlier crankshaft angle; in particular, it is moved closer to the ignition TDC if the center of combustion is after the ignition TDC. Correspondingly, a retarded change in the center of combustion means that the center of combustion is shifted towards a later point in time or later crankshaft angle relative to the firing TDC, in particular being further away from the firing TDC if the center of combustion is later in time than the firing TDC lies. The center of combustion is typically associated with an ignition point, in particular it can be specified by selecting the ignition point. A shift in the center of combustion to an earlier or later point in time can be brought about in particular by changing the ignition point. In this case, a change in the ignition point to an earlier or a later point in time corresponds in an analogous manner to a change in the center of combustion to an earlier or later point in time, although the point in time to the ignition is typically before the ignition TDC. Thus, advancing spark timing means increasing the distance of spark timing from firing TDC, while retarding spark timing means bringing spark timing closer to firing TDC.

A change in the lambda value towards higher values means a leaner combustion air/fuel mixture in the combustion chamber of the internal combustion engine. A change in the lambda value towards lower values means that the combustion air/fuel mixture is enriched. Leaning means a change in the combustion air/fuel ratio in favor of the combustion air mass. Enrichment means a change in the combustion air/fuel ratio in favor of the fuel mass.

The fact that the lambda value is raised in step f) means that the lambda value is changed to higher values.

A gradual change in the lambda value and/or the center of combustion means, in particular, a change by a respective predetermined change value, in particular a respective predetermined increment or decrement.

The internal combustion engine is preferably designed as a single-cylinder engine, in particular a single-cylinder test engine. However, it can also have a plurality of combustion chambers.

According to a development of the invention, it is provided that after step a) and before step b) the internal combustion engine is operated under constant conditions for a predetermined time interval with the starting value pair. The internal combustion engine is additionally operated after step c) and before step d) at the starting value pair for a predetermined time interval under constant conditions. The internal combustion engine is preferably additionally operated after step e) and before step f) at the limit combustion center of gravity at the second limit for a predetermined time interval under constant conditions. The operation of the internal combustion engine under constant conditions enables the operation of the internal combustion engine to be stabilized under the respective operating conditions, so that it is ensured that meaningful values for the characteristics map can be obtained with the exclusion of transient effects.

Constant operation of the internal combustion engine means in particular that all parameters, in particular at least all parameters that can be influenced, are kept constant during operation of the internal combustion engine.

According to a further development of the invention, it is provided that the first limit is selected from a predetermined exhaust gas temperature limit and a predetermined latest limit for the center of combustion. The first limit can therefore be defined in particular as a predetermined maximum temperature for the exhaust gas of the internal combustion engine, which should not or must not be exceeded. On the other hand, it can also be defined as a predetermined latest limit for the focal point of combustion, ie a latest point in time or latest crankshaft angle at which the focal point of combustion should or may lie. Compliance with these conditions ensures, in particular, damage-free operation of the internal combustion engine.

According to one development of the invention, it is provided that the second limit is selected from a group consisting of a predetermined knock limit, a predetermined spark plug temperature limit, and a predetermined earliest limit for the combustion focus. Compliance with these conditions also ensures damage-free operation of the internal combustion engine.

The predetermined knock limit is preferably defined as a predetermined limit value for a knock index, which preferably consists of a particular high-resolution combustion chamber pressure signal is determined. The combustion chamber pressure signal is preferably filtered beforehand. In particular, a mean value of all amplitudes of individual work cycles of the internal combustion engine can be used as a value for the knock index that is preferably available in real time.

The predetermined spark plug temperature limit is in particular a predetermined maximum temperature, measured at a spark plug of the internal combustion engine or in an area surrounding the spark plug.

The predetermined earliest limit for the center of combustion is—similar to the predetermined latest limit for the center of combustion—a predetermined earliest point in time or earliest crankshaft angle that the center of combustion may assume.

According to one development of the invention, it is provided that the third limit is a predetermined misfire limit. In particular, compliance with the corresponding limit enables the internal combustion engine to run smoothly and without misfiring. Combustion stability is preferably evaluated to determine the predetermined misfire limit. For this purpose, in particular, a coefficient of variation (COV) of the indicated mean pressure p _mi is determined, this value also being referred to as PMI_COV for short. This variation coefficient is preferably determined as follows: an indicator system uses a combustion chamber pressure sensor, in particular a piezo pressure quartz, to record the combustion chamber pressure curve in the combustion chamber over time or the crankshaft angle. The indicated mean pressure p _mi corresponds to the indicated work related to the displacement Vh: $$p_{mi}=\frac{\oint p\left(V\right)\mathrm{i.e}V}{V_H}.$$

Here p(V) is the combustion chamber pressure as a function of the current combustion chamber volume V.

wobei n die Anzahl der durchgeführten Messungen, i ein Laufindex über die einzelnen Messungen, p_mi,i der bei der durch den Laufindex i gekennzeichneten Messung erfasste Wert des indizierten Mitteldrucks p_mi, und p _mi der Mittelwert des indizierten Mitteldrucks p_mi ist.The coefficient of variation PMI_COV is now defined as the quotient of the standard deviation p _mi,s and the expected value - or, in the case of an existing series of measurements, the mean value of the indicated mean pressure p _mi . The standard deviation p _mi,s is defined as: $$p_{mi,s}=\sqrt{\frac{1}{1-\text{n}}{\displaystyle {\sum }_{i=1}^{\text{n}}\left(p_{mi,i}-\overline{p_{ml}}\right)},}$$

where n is the number of measurements carried out, i is a running index for the individual measurements, p _mi,i is the value of the indicated mean pressure p _mi recorded in the measurement identified by the running index i, and p _wed is the mean of the indicated mean pressure p _mi .

Accordingly, the coefficient of variation PMI_COV is now defined according to the following equation (3): $$\text{PMI\_COV}=\frac{P_{mi,s}}{\overline{P_{ml}}}.$$

The predetermined misfire limit is preferably defined by a predetermined limit value for the coefficient of variation PMI_COV defined in this way.

According to a development of the invention, it is provided that the predetermined pollutant concentration is a predetermined nitrogen oxide concentration, in particular a total nitrogen oxide concentration. The predetermined pollutant concentration is preferably 500 mg/m _n ³ (mg per standard cubic meter). A characteristic map line can thus be defined in a particularly advantageous manner, along which the characteristic map can be scanned at least approximately in the center of the characteristic map.

According to a development of the invention, it is provided that the lambda value and the center of combustion are varied under the additional condition that a combustion chamber pressure gradient does not exceed a predetermined combustion chamber pressure gradient limit value. This also ensures safe operation of the internal combustion engine. The combustion chamber pressure gradient is in particular a time derivative of the combustion chamber pressure curve or—equivalently—a derivative based on the crankshaft angle.

According to a development of the invention, it is provided that the lambda value and the center of combustion are varied under the additional condition that a combustion chamber peak pressure does not exceed a predetermined combustion chamber peak pressure limit value. This also advantageously ensures safe operation of the internal combustion engine, in particular without mechanical damage to the combustion chamber. The combustion chamber peak pressure is a pressure maximum of the combustion chamber pressure profile in the combustion chamber.

According to a development of the invention, it is provided that the lambda value is varied by changing the combustion air mass supplied to the combustion chamber of the internal combustion engine. Alternatively or additionally, the lambda value is preferably varied by changing the fuel mass supplied to the combustion chamber. In particular, it is possible that both the supplied Ver combustion air mass and the supplied fuel mass can be changed. Both variables are typically easily adjustable on a test bench.

Alternatively or additionally, it is preferably provided that the center of combustion is varied by changing the ignition point. This is also easily possible on a test stand, especially if the internal combustion engine is designed as a spark-ignited internal combustion engine, in particular by means of electrical spark ignition, corona ignition, laser ignition, pilot ignition, or spark-ignited in another suitable manner. The ignition point at least essentially determines the center of combustion.

Finally, the object is also achieved by creating a control unit for controlling a test stand for an internal combustion engine, the control unit being set up to carry out a method according to the invention or a method according to one of the previously described embodiments. In connection with the control device, the advantages already mentioned are realized in particular.

The pollutant concentration in the exhaust gas is measured in particular. In this way it can easily be determined whether the predetermined pollutant concentration is maintained.

Accordingly, the conditions for the first border, the second border and the third border are measured. In particular, at least one parameter is preferably measured, selected from a group consisting of the combustion air mass, the fuel mass, the combustion chamber pressure profile, the exhaust gas temperature, the spark plug temperature, and a pollutant concentration, in particular nitrogen oxide concentration, in particular total nitrogen oxide concentration. The center of combustion is determined in particular on the basis of the combustion chamber pressure profile.

• 1 eine schematische Darstellung eines Ausführungsbeispiels eines Prüfstands für eine Brennkraftmaschine mit einem Ausführungsbeispiel eines Steuergeräts, und
• 2 eine schematische Darstellung eines im Rahmen einer Ausführungsform des Verfahrens zu vermessenden Kennfelds.

The invention is explained in more detail below with reference to the drawing. show:

• 1 a schematic representation of an exemplary embodiment of a test bench for an internal combustion engine with an exemplary embodiment of a control unit, and
• 2 a schematic representation of a map to be measured as part of an embodiment of the method.

1 shows a schematic representation of an embodiment of a test stand 1, on which an internal combustion engine 3 is arranged. The test stand 1 is set up to measure a characteristic map of the internal combustion engine 3 . In particular, the test stand 1 has a control unit 5 which is set up to carry out an embodiment of a method for automated measurement of the characteristic diagram of the internal combustion engine 3 which is explained in more detail below. The internal combustion engine 3 has at least one combustion chamber 4 . It is preferably designed as a single-cylinder test engine.

2 shows a schematic representation of a map of the internal combustion engine 3, which is spanned by a combustion center, which is represented here by an ignition point ZZP as a variable, by which the combustion center is determined. The map is also spanned by a combustion air/fuel ratio, which is also referred to as the lambda value λ.

As part of the method, a starting value pair 7 is determined for the map from a starting value for the center of combustion or ignition point ZZP and a starting value for the lambda value λ, with this starting value pair 7 approximately in an estimated middle of the map on a predetermined Pollutant concentration in the exhaust gas of the internal combustion engine 3 defined map line 9 is located.

Starting from the start value pair 7 - see b) - the combustion focus, i.e. the ignition point ZZP, is gradually retarded and the lambda value λ is changed to lower values, under the condition that the predetermined pollutant concentration along the map line 9 is maintained. Retarding the center of combustion means here that an in 2 the distance between the ignition point ZZP and an ignition TDC of the internal combustion engine 3, which is plotted on the abscissa as a time or measured in crankshaft angle degrees (°KW), is reduced.

Corresponding values for the center of combustion and the lambda value λ along the characteristic curve 9 defined by the predetermined pollutant concentration are stored.

The change in the center of combustion and in the lambda value λ is repeated until a first limit 11 is reached.

Then—see d)—starting from the pair of starting values 7, the center of combustion is gradually advanced and the lambda value λ is changed to higher values under the condition that the predetermined pollutant concentration is lower is maintained along the characteristic curve 9 . Advancing the center of combustion means here that the ignition point ZZP is changed at greater distances from the ignition TDC of the internal combustion engine 3 . Corresponding values for the combustion center and the lambda value λ on the characteristic curve 9 defined by the predetermined pollutant concentration are stored. The corresponding changes in the center of combustion and the lambda value λ are repeated until a second limit 13 is reached, with the center of combustion at the second limit 13 being stored as limit center of combustion 15, determined here in particular by a first specific ignition point ZZP1.

The lambda value λ is now—see f)—incrementally increased until a third limit 17 is reached in a first limit value pair 19 from the center of combustion and the lambda value λ. The first limit value pair has the limit combustion center 15 as the value for the combustion center.

Starting from the first limit value pair 19 - see g) - at the third limit 17 the center of combustion is gradually retarded and the lambda value λ is changed to lower values along the third limit 17, corresponding values for the center of combustion and the lambda value λ being stored , until the first limit 11 is reached.

Now—see h)—the lambda value λ is gradually changed to lower values along the first limit 11 , and if there is a deviation from the first limit 11 , the center of combustion is varied in order to remain at the first limit 11 . Corresponding values of the combustion center and the lambda value λ are stored. This is carried out until the second limit 13 is reached in a second limit value pair 21 from the center of combustion and the lambda value λ.

Now—see i)—starting from the second limit value pair 21, the center of combustion is gradually advanced and the lambda value λ is changed to higher values along the second limit 13, with corresponding values of the center of combustion and the lambda value λ being stored until the third limit 17 is reached at an intersection 23, where the second boundary 13 and the third boundary 17 intersect.

Now—see j)—starting from the point of intersection 23, the center of combustion is gradually retarded along the third limit 17 and the lambda value λ is changed to lower values, with corresponding values of the center of combustion and the lambda value λ being stored until the center of combustion is on the third limit 17 is equal to the limit center of combustion 15, which is the case in turn in the first limit value pair 19.

The internal combustion engine 3 is preferably operated after the determination of the starting value pair 7 and before the gradual retarding of the center of combustion and of the lambda value λ to lower values for the starting value pair 7 for a predetermined time interval under constant conditions. Constant conditions mean in particular that all parameters of the operation of internal combustion engine 3, at least all parameters that can be influenced, are kept constant. The internal combustion engine 3 is also after the first reaching of the first limit 11 - starting from the pair of starting values 7 - and before the gradual change in the center of combustion to the advance and the lambda value λ to higher values - according to d) - again at the pair of starting values 7 for a predetermined time interval operated under constant conditions.

The internal combustion engine 3 is also preferred after the first reaching of the second limit 13 - starting from the starting value pair 7 - and before the gradual increase according to f) of the lambda value λ while keeping the center of combustion constant at the limit center of combustion 15 - at the second limit 13 operated under constant conditions for a predetermined time interval.

The first limit 11 is preferably selected from a predetermined exhaust gas temperature limit and a predetermined latest limit for the center of combustion.

The second limit 13 is preferably selected from a group consisting of a predetermined knock limit, a predetermined spark plug temperature limit, and a predetermined earliest limit for the center of combustion.

The third limit 17 is preferably a predetermined misfire limit.

The predetermined pollutant concentration is preferably a predetermined nitrogen oxide concentration, total nitrogen oxide concentration, preferably 500 mg/m _n ³ .

The lambda value λ and the center of combustion are preferably varied under the additional condition that a combustion chamber pressure gradient does not exceed a predetermined combustion chamber pressure gradient limit.

The lambda value λ and the center of combustion are preferably varied under the additional condition—in particular in addition to the aforementioned additional condition—that a peak combustion chamber pressure does not exceed a predetermined peak combustion chamber pressure limit value.

The lambda value is preferably varied by changing a combustion air mass supplied to the combustion chamber 4 of the internal combustion engine 3 and/or a fuel mass supplied to the combustion chamber 4 .

Alternatively or additionally, the center of combustion is preferably varied by changing the ignition point ZZP.

Claims (10)

Method for the automated measurement of a map of an internal combustion engine (3) on a test bench (1), wherein a) for the map to be measured, spanned by a combustion center and a combustion air/fuel ratio, i.e. a lambda value (λ), a start value pair (7) for the combustion center and the lambda value (λ) in an estimated center of the map a predetermined concentration of pollutants in the exhaust gas of the internal combustion engine (3) is determined, wherein b) starting from the start value pair (7), the combustion center of combustion is gradually retarded and the lambda value (λ) is changed to lower values under the condition that the predetermined pollutant concentration is maintained, with corresponding values for the combustion center of combustion and the lambda value (λ) be stored until c) a first limit (11) is reached, wherein d) starting from the pair of starting values (7), the combustion center is gradually advanced and the lambda value (λ) is changed to higher values under the condition that the predetermined pollutant concentration is maintained, with corresponding values for the combustion center and the lambda value (λ) be stored until e) a second limit (13) is reached, the center of combustion at the second limit (13) being stored as the limit center of combustion (15), wherein f) the lambda value (λ) is gradually increased until a third limit (17) is reached in a first pair of limit values (19) from the center of combustion and the lambda value (λ), wherein g) starting from the first pair of limit values (19) at the third limit (17), the center of combustion is gradually changed retarded and the lambda value (λ) is changed to lower values along the third limit (17), with corresponding values for the center of combustion and the lambda value (λ) are stored until the first limit (11) is reached, where h) the lambda value (λ) is gradually changed to lower values along the first limit (11) and, if there is a deviation from the first limit (11), the center of combustion is varied in order to remain at the first limit (11), with corresponding values of the center of combustion and the lambda value (λ) are stored until the second limit (13) is reached in a second limit value pair (21) from the combustion center and the lambda value (λ), wherein i) starting from the second pair of limit values (21), the center of combustion is gradually changed further and the lambda value (λ) to higher values along the second limit (13), with corresponding values of the center of combustion and the lambda value (λ) being stored, until the third limit (17) is reached, where j) along the third limit (17) the center of combustion is gradually retarded and the lambda value (λ) is changed to lower values, with corresponding values of the center of combustion and the lambda value (λ) being stored until the center of combustion is on the third limit (17) is equal to the limiting center of combustion (15). procedure after claim 1 , characterized in that the internal combustion engine (3) - after step a) and before step b), as well as after step c) and before step d), in each case at the starting value pair (7), and preferably - after step e) and prior to step f) at the limit combustion center of gravity (15) at the second limit (13) is operated under constant conditions for a predetermined time interval in each case. Method according to one of the preceding claims, characterized in that the first limit (11) is selected from a predetermined exhaust gas temperature limit and a predetermined latest limit for the center of combustion. Method according to one of the preceding claims, characterized in that the second limit (13) is selected from a group consisting of a predetermined knock limit, a predetermined spark plug temperature limit, and a predetermined earliest limit for the combustion center. Method according to one of the preceding claims, characterized in that the third limit (17) is a predetermined misfire limit. Method according to one of the preceding claims, characterized in that the predetermined pollutant concentration is a predetermined nitrogen oxide concentration, in particular total nitrogen oxide concentration, preferably 500 mg per standard cubic meter. Method according to one of the preceding claims, characterized in that the lambda value (λ) and the center of combustion are varied under the additional condition that a combustion chamber pressure gradient does not exceed a predetermined combustion chamber pressure gradient limit value. Method according to one of the preceding claims, characterized in that the lambda value (λ) and the center of combustion are varied under the additional condition that a combustion chamber peak pressure does not exceed a predetermined combustion chamber peak pressure limit value. Method according to one of the preceding claims, characterized in that - the lambda value (λ) is varied by changing a combustion air mass supplied to a combustion chamber (4) of the internal combustion engine (3) and/or a fuel mass supplied to the combustion chamber (4), and/or that - the center of combustion is varied by changing an ignition point (ZZP). Control unit (5) for controlling a test bench (1) for an internal combustion engine (3), the control unit (5) being set up to carry out a method according to one of Claims 1 until 9 .

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